The present invention relates to fiber-optic sensors, and particularly to distributed fiber-optic sensor arrays which utilize a short coherence length light source.
Over the past few years, fiber-optic devices have been actively studied and developed for use in various sensing applications in a wide range of fields. One reason for this interest is the sensitivity of optical fibers to environmental conditions which surround them. For example, factors such as temperature, pressure, and acoustical waves directly affect the light transmitting characteristics of optical fiber. These changes in the optical fiber produce a change in the phase of light signals traveling in the fiber. Thus, a measurement of the change in phase of optical signals which have been transmitted through that fiber is representative of changes in those environmental conditions which have affected the fiber.
Recently, particular efforts have been directed to the development of systems having sensors organized in arrays, so that a number of sensors can utilize light from a single source, and provide environmental information at a common detection location. Ideally, such an array would consist of a fiber input bus which would carry light to a set of sensors. Each sensor would imprint information about the environment to this optical carrier. An output fiber bus would then collect this information and bring it back to a central processing location, where information obtained from any selected one of the sensors could be readily identified and analyzed.
The goal of these development efforts is to produce sensor arrays which could be used for specific applications such as monitoring rapidly changing environmental conditions. For example, such sensor arrays could be used to detect acoustic waves in order to determine the source location and acoustical characteristics of those waves. For many such applications, it may be necessary to space the arrays over a relatively large area. In these situations, the replacement of electrical lines by fiber optics, for example, would overcome problems such as electrical pickup, cable weight, and safety hazards associated with the use of those electrical lines. Even when the sensor is used in limited space, the removal of electronics and bulk optics components generally should provide improved system performance due to reduced noise. On the other hand, replacement of long electrical lines by optical fibers creates a problem in preventing or removing any influence of environmental conditions on the non-sensor portions of the system. This, therefore, becomes an important design consideration.
Of course, the primary design consideration in developing a sensor array is the method by which information from each sensor can be separated for individual identification from among all of the information arriving at the central processing location on the single data stream. Distributed sensing systems developed previously have generally applied one of two approaches for separating information of an individual sensor from a single data stream. The first approach comprises time-division multiplexing of the sensor outputs, as is described by A. R. Nelson and D. H. McMahon, "Passive Multiplexing Techniques For Fiber-Optic Sensor Systems," I.F.O.C., Page 27, March, 1981. In time-division multiplexing, the optical input most generally is pulsed so that the input signal comprises a pulse waveform. As a result each sensor produces a pulse which, as a consequence of the system geometry, is separated in time from the other sensor signals. Specifically, the optical input pulse communicated through each sensor is placed on the output fiber by each of the sensors at a different time. By controlling the relative position of the sensors, interleaving of the pulse signals may be accomplished as the signals are multiplexed from the sensors onto a return fiber bus. These interleaved pulse signals are then carried back to the central processing location where demultiplexing and further signal processing occur.
One problem which is inherent with this type of system is that the frequency at which the sensors may be monitored becomes limited by the number of sensors. Specifically, it is noted that a second pulse may not be transmitted from the optical source until a certain amount of time has passed. If the second pulse were transmitted through the sensor nearest the source before the optical signals from all sensors had passed the output terminal of that sensor, it is possible that signals resulting from the second pulse could pass through the first sensors in the array and be placed on the return bus prior to the passing of optical signals produced from sensors near the end of the array. This would, of course, prevent the demultiplexing and signal processing equipment from determining the relationship between the pulse signal received and its associated sensor. Such systems are, therefore, often not useful in applications requiring rapid repeated sensing of environmental conditions by each of the sensors in the array.
The second approach which has been used for separating each sensor's information from the single data stream has been to frequency-division multiplex the sensor outputs, in the manner described by I. P. Giles, D. Uttam, B. Culshaw, and D. E. N. Davies, "Coherent Optical-Fibre Sensors With Modulated Laser Sources," Electronics Letters, Vol. 19, Page 14, (1983). This approach is accomplished by frequency ramping the optical source and arranging the array geometry so that the transit time of the light from the source to a sensor and back to the central location is unique for each sensor. In this case, the array output is mixed with the source's present output, thereby producing a unique central frequency for each sensor. The environmental information is carried in the sidebands about this central frequency.
One particular problem with the above-described system involves the "fly back" period when the periodic ramp signal is reset from its maximum to its minimum position. This fly back period comprises a time when system operation may not occur, since no ramp signal is present, and no meaningful results would be produced. This places some limit on the rate at which environmental conditions may change and still be reliably monitored by the sensor system.
Another problem with this particular system is that the number of sensors which may be used in the array or the frequency range of the signals to be detected are limited based on the range of FM frequencies which are utilized in the ramp signal, and on the period of the ramp signal. More specifically, since a different central frequency is produced for each sensor, the amount of difference between each such central frequency and the overall range of frequencies within which these central frequencies are contained dictates the number of sensors which may be utilized. Equivalently, the number of sensors, together with the overall range of frequencies determine the maximum difference between central frequencies, and hence the maximum environmental frequencies which may be detected. The range of frequencies is, of course, determined by the slope and period of the ramp signal.
Another limitation experienced by both of the approaches described above is that they are limited to longer coherence length sources, since they require the use of interference between the original source signal and the signal produced by the sensor in order to identify the desired environmental conditions. Thus, both of those systems use either pulsed or ramped coherent sources for producing the optical signal.
The idea of using a short coherence length source to separate signals returning from a series of sensors has been proposed by S. A. Al-Chalabi, B. Culshaw, and D. E. N. Davies, "Partially Coherent Sources In Interferometric Sensors," Proceedings of the First International Conference On Optical Fibre Sensors, (I.E.E.E.), Page 132, April, 1983. That reference discloses the use of a series of remote Mach-Zehnder interferometers with the difference in the length of the arms in each interferometer being greater than the coherence length of the source, so that there is no interference signal on the output of the interferometers. Two optical fibers connect the outputs of each interferometer to the inputs of the next interferometer. The output fibers of the last sensing interferometer are connected to the input ports of a single reference interferometer having a detector positioned on one of its output ports. The reference interferometer is constructed from bulk optical components and configured so that the delay in one of its arms is variable. The receiver varies the delay in the indicated arm, thereby effectively varying the length of the optical path through that arm to detect signals from each of the various interferometric sensors in the system. The reference interferometer must be constructed from bulk optical components rather than fiber so that its arm length can be varied enough to accommodate a significant number of sensors.
From the above, it becomes apparent that the Al-Chalabi et al. reference does not disclose a system which may continuously monitor each of the various sensors in a distributed system. Rather, the Al-Chalabi et al. system merely detects the environmental conditions sensed by any single sensor at a given time. The environmental conditions on all the sensors can be detected only periodically by monitoring each sensor sequentially. The frequency with which this can be done is limited by the speed with which the length of the variable arm of the receiver can be varied.
Another problem with this system is that in such a system the .pi./2 phase shift which occurs when light couples between two fibers becomes important. Light from one input port of a sensing interferometer enters the longer arm delayed by .pi./2 relative to light entering the shorter arm. Light from the second input port enters the shorter arm with a relative delay of .pi./2 radians. This difference in relative delays leads to cancellation between the signals associated with light entering each of the two input ports, so that if all couplers are set to a coupling efficiency of 50%, then only the first sensor will produce any signal at all.
Based on the above, it would be an important improvement in the art to provide a sensing system and technique for multiplexing a plurality of remote sensors without being subject to the above-identified restrictions which are inherent in the time-division and frequency-division multiplexing schemes used in the past. Thus, the improved system should optionally be time-independent, so that substantially continuous monitoring of each of the sensors is possible. Such a system should provide for operation without requiring use of electronics or active devices in the environmental sensing region. Preferably, such a system should permit use of any of a wide range of optical sources, and should be both simple and economical to produce and use in actual application.